[Aavso-photometry] Linearity test puzzle

[Vello Tabur]

I've been testing the linearity of my camera (SBIG ST8XE) using a LED + 
voltage regulator, as per the circuit in AIP v1 (pg 201).
Ten exposures (30sec each) were taken either side of a linearity test to 
assess LED stability during the test.  The plot below shows a gradual 
increase in LED brightness of 0.24% over a period of one hour. A curious 
systematic feature appears on the plot: the first exposure of each group 
shows an rapid increase in flux relative to the other exposures. It 
occurs in each group and the same effect occurs in tests conducted on 
other nights.  Any suggestions as to what could be causing this?